ACADEMY OF BCIE.NCM] POWELL'S SURVEY 87 



sequent studies have confirmed this latter opinion, and given some support to the view that 

 even if the solidified basic rocks of surface volcanoes are denser than the solidified acidic rocks 

 of laccoliths, the molten basic rocks may, before they lost their included gases and vapors 

 during eruption, have been less dense than the molten acidic rocks in their subterranean cis- 

 terns. Hence the relation of densities now obtaining may be due in part at least to the loss of 

 a good share of initial gases in the surface rocks. In other words, the contraction and density 

 increase of the surf ace basic rocks in cooling at the time of their eruption may have been greater 

 than that of the buried acidic rocks; but H is generally felt that Gilbert carried this principle 

 too far. 



However, he cited "a fact of observation which tends to sustain the view that the laccolitic 

 rocks of the Henry Mountains contracted less in cooling than the volcanic." Prismatic struc- 

 ture is produced by contraction in cooling; and as it is absent in the Henry Mountain trachytes, 

 their contraction must have been small; conversely, its presence in many basic volcanic rocks 

 indicates that their contraction was relatively great. But if the hydrostatic law holds good, the 

 acidic rocks of the subterranean laccoliths may have contracted by a considerable amount; for 

 if those rocks really were, when molten, only slightly denser than the strata above them (2.3), 

 then their volume when molten must have been about one-tenth greater than when solid ; and 

 to this it may be added that, under the same hypothesis, the increase in density of surface 

 volcanic rocks on passing from the molten to the solid state must have been greater still. Thus 

 it is possible that if Gilbert's hydrostatic theory of laccolithic intrusion proves to be measure- 

 ably true, it may, by means of a comparison of the densities of stratified rocks, laccoliths, and 

 volcanoes, lead to a better determination of the density of both intrusive and extrusive igneous 

 rocks when they were in a molten state and of their increase of density on solidifying, than is 

 obtainable by experiment; for artificial conditions in the laboratory can hardly imitate natural 

 conditions in a volcanic vent, much less those in an underground laccolithic reservoir. But it 

 must be added at once that laccoliths later found in other parts of the Cordilleran region have 

 varied compositions and are thought by their investigators to discredit Gilbert's view that 

 density of magma was an important factor in the formation of laccoliths. Yet none of these 

 investigators has published so critical an analysis of the factors of penetrability and density 

 as Gilbert did. 



MECHANICS OF LACCOLITHIC INTRUSION 



The physical conditions of intrusion being thus outlined, the mechanical changes that accom- 

 panied the intrusions are examined. It should be here recalled that the average form of the 

 Henry Mountains laccoliths is that of a low arched dome with a rapid marginal descent; and that 

 the covering strata therefore rise in a steep-sided, flat-topped quaquaversal arch; also that, 

 although the flat tops and the flexed sides of the arched beds are broken by radial fissures occupied 

 by dikes, the flexed sides are not fractured on circumferential lines; hence the central covering 

 strata must have been increased in radial measure by stretching during deformation. The 

 stretching is explained as due to an extension or outward squeezing of the immediately overlying 

 strata between the upward pressure of the intruding trachyte and the downward weight of the 

 higher strata. An interesting corollary follows : Any fractures that were made in the arch must 

 have been instantly filled with intrusive dikes, and no empty fissures were left to be slowly filled 

 afterwards with mineral veins; and.it is for this reason that the Henry Mountains have no 

 attraction to the miner (83). 



The cause of the steep dips around the margin of the flat-topped laccolithic domes is next 

 sought for; and it is particularly here that Gilbert's analysis of the problem has seldom been 

 quoted, perhaps seldom understood, so fully as it deserves. It is probable that some of the 

 masses are more flat-topped than others; if so, it is the typical flat-topped domes that are 

 especially considered. It was conceived that when lateral movement of the rising magma 

 began, a comparatively thin horizontal sheet was formed, which would be nearly circular if the 

 ease of lifting the strata were about the same in all directions outward from the supplying 

 c himn ey or neck. Consideration was then given to the distribution of pressure exerted by the 

 intruding magma, the thickness of the overlying strata being constant. The total upward 



